Motor vehicle manufacturers today employ a variety of suspension systems for isolating the vehicle body from irregularities in the terrain over which the vehicle travels. Most such suspension systems may be categorized as either passive or active. Passive suspension systems typically react to forces via the inherent mechanical characteristics of a spring and damper combination connected between the vehicle body and wheels. Active suspension systems, in contrast, typically react to forces via electronic control of hydraulic or pneumatic fluid actuators normally connected between the vehicle body and wheels.
In addition to isolating the vehicle body from terrain irregularities, suspension systems have also been employed to stabilize the tendency of the vehicle body to tilt, or "roll," relative to the vehicle wheels during various vehicle maneuvers such as acceleration, deceleration, or cornering. U.S. Pat. No. 5,161,822 issued to Lund ("the Lund '822 patent") and U.S. Pat. No. 5,106,120 issued to Di Maria ("the Di Maria '120 patent") each disclose roll-control systems having a pair of hydraulic cylinder and piston assemblies connected between an anti-roll bar and an axle. The Lund '822 and Di Maria '120 patents thereby maintain the vehicle body in a substantially level position relative to the vehicle wheels, regardless of the force seeking to upset that position.
Nevertheless, vehicle suspension systems adapted for roll control still suffer from a variety of problems. First, as the vehicle is driven along a straight line path at a constant speed, the forces seeking to upset the level position of the vehicle body are generally negligible. However, if these forces become substantial, roll control systems would ideally permit fluid pressure therein to vary, or "float," so as not to interfere with the remaining portions of the vehicle suspension system working to isolate the vehicle body from terrain irregularities. However, the most actuators used in known roll control systems, such as the hydraulic cylinder and piston assemblies in the Lund '822 patent, are actively subjected to a fluid pressure at all times and are therefore unable to "float" as the vehicle is driven along a straight line path.
Moreover, roll control systems may include solenoid activated valves to control fluid pressure in the system actuators. Such valves have a de-energized position and at least one energized position, which are used to establish various fluid pressure levels in the system actuators depending upon the direction in which the vehicle is cornering. In the event of an electrical power failure, such roll control systems would ideally provide "firm" control, or a fluidly locked condition simulating a passive system. Under such circumstances, however, solenoid activated valves, shuttle to their de-energized positions, which permit fluid flow to and from the system actuators, as in the roll-control system of the Di Maria '120 patent. As a result, the active pressure control in the system actuators is lost and the roll control system "floats" rather than providing the "firm" control desired.
Thus, in addition to compensating for vehicle roll during cornering, an improved roll control system and method would provide "floating" roll control when the vehicle is driven along a straight line path, and "firm" roll control in the event of an electrical power failure. More specifically, an improved roll control system would be fluidly neutral when the vehicle is driven along a straight line path so as not to interfere with the remaining portions of the vehicle suspension system. Such a roll control system and method would also be fluidly locked in the event of an electrical power failure.